World : High-Energy Cosmic Rays Come from Outside Our Galaxy

Giant observatory announces long-awaited result

The Pierre Auger Observatory in Argentina finally has solid evidence that the most energetic particles in nature come from sources outside the Milky Way. Scientists have suspected this for decades, but weren’t able to confirm it—until now.

“For the first time, we have proof that the highest-energy cosmic rays are of extragalactic origin,” says Alan Watson, a UK astronomer and co-founder of the observatory. The result comes as a relief to the researchers, after previous claims regarding their origin made ten years ago by the Pierre Auger Collaboration subsequently turned out to be premature.

The international team analysed 12 years’ worth of data, and found that particles in the upper range of energies were more likely to come from a region of the sky outside the Milky Way’s disk. That asymmetry is roughly consistent with the distribution of neighbouring galaxies, the researchers report in the 22 September issue of Science.

The study does not pinpoint individual sources of the cosmic rays, or explain how they reach their highest energies. But the researchers hope that it is a first step towards understanding their origins.